Do you mean to say that an employee who would take a Half-day Sick Leave on March 8 and then a Full Day Sick Leave on March 15 could not take more thant 1.5 additional Sick Leave days in the month of March?
To restrict Data Entry, your may be able to create a "Monthly Sick Days" Quota with a Monthly Allowance of 3 days, but it's an option I have never tried. You could also ask an ABAP Programmer to create an Include that would validate the current entry against the Monthly Cumulation (in Table SALDO or a Custom Table) and number 3 (or a Constant from V_T511K) before being able to "Save" the Absence.
To generate an ERROR during Time Evaluation, you could create a pcr to check if the Daily Sick Days Leave "days" plus the Cumulated Monthly Sick Leave "days" exceed the value of a Constant from V_T511K.